Some of the metabolic parameters which might contribute to the high incidence of tumors observed in old age will be studied in experimental animals. The modifications of these parameters after long-term dietary restrictions or physical training will also be determined. The purpose of this study is to determine the correlation between the age related changes in (1) body weight and composition, (2) efficiency of energy utilization and (3) maintenance requirements, and tumor development. The influence of dietary manipulation and physical exercise on these age related changes will be studied in order to evaluate (1) the mechanism by which long-term dietary restriction decreases tumor incidence and (2) the possibility of obtaining the same effects by physical exercise. Rats of the following age groups will be considered: 21, 60, 100, 300, 600 days. The efficiency of energy utilization for growth will be determined by feeding 4 groups of rats a semipurified 24% casein diet daily for 21 days at 4 different levels of energy intake. On the 21st day the animals will be killed; changes in body weight will be correlated to the metabolizable energy intake (MEI) per metabolic body size (MBS); the slope of the regression line will indicate the efficiency of energy utilization for body growth. Body composition will be analyzed by freeze-drying and solvent extraction of the carcasses. Changes in lean and fat mass will be correlated to MEI/MBS, the slope of the lines obtained by regression analysis will indicate the coefficients of energy utilization for lean and adipose mass deposition. The x-axis intercepts of the regression lines will indicate the MEI/MBS for maintenance of each body compartment. The rates of tumor growth will be determined by inoculating a cell suspension of a 7800 Morris hepatoma in the hind legs of some animals, harvesting and weighing the tumor after 21 days. Efficiency of energy utilization, maintenance requirements and body composition of tumor bearing animals will be measured. Dietary restriction will be obtained by feeding each age group restricted amounts of a 24% casein diet for 30 days. The influence of physical exercise will be studied by training the animals of each age group in motorized cages for 30 days.